1. Field of the Invention
This invention relates to a portable electronic scale of minimal thickness and weight which can be easily carried in a bag, stored in a cabinet or hung on a wall, for measuring the weight of persons or objects.
2. Description of the Prior Art
Most small scales, such as those used for personal weight measurement, require that the person stand or the object be placed on a flat and rigid load-bearing plate, which rests on a set of levers touching the plate from below at a number of discrete points. The levers rest on a second load-bearing plate which is, in turn, placed on a flat floor. The levers are connected in such manner that when a load is placed on the top plate, the levers exert a load at a single point which is equal to the total load on the top plate. This load is then measured, either by balancing it against a known load as in the scales commonly found in clinics, or by applying that load to a mechanically deformable element, such as a spring or a beam, and measuring the deformation. Common "portable" bathroom scales usually measure the movement of a spring by rotating a dial. Newer scales measure the deformation of a spring or an alternative mechanically-deformable element electronically, with strain gauges or capacitors, and display the measure digitally, usually with a light-emitting diode (LED).
The lever mechanisms, the two (or more) load-bearing plates and the power requirements for the LED's usually impose discrete weight and thickness requirements on most known scales, making them too heavy and too bulky to be easily transported from place to place. The smallest scales for personal weight measurement, for example, weigh several pounds and are about one to three inches in thickness. This makes it difficult for them to be carried so that an individual can watch his or her weight while away from home. It also makes it difficult to lift the scales and store them in a cabinet or hang them on a wall when floor space (such as in a bathroom) is limited.
A different principle for constructing a scale which does not require any lever mechanism for the mechanical transfer of loads for measurement at a single point has been in operation in some industrial scales for some time. It involves placing a rigid plate on a plurality of mechanically deformable elements, connecting force transducers such as strain gauges to these elements and summing up the electrical signals from these transducers in a Wheatstone bridge balancing circuit to obtain a measure proportional to the total load on the plate. Because the load on the plate is the sum of all the loads on the elements, this measure is the same regardless of the distribution of the load on the plate.
Ostrelich U.S. Pat. No. 4,355,692 cites several U.S. patents for industrial scales operating on this principle, namely U.S. Pat. Nos. 4,150,729; 3,949,822; 3,966,003; and 4,146,100. Ostrelich then describes an application of this principle to a small scale for weighing persons, proposing to reduce the cost of manufacturing by replacing the more-common strain gauge transducers with a thick film resistor. He describes a weighing scale in which a load impressed on a plurality of spaced individual transducers is electrically added to indicate a total weight of the load. While not claiming that application of this principle for small scales for weighing persons is new, he does state that the application of the thick film resistors makes it possible to produce a scale of a very limited vertical dimension. The embodiments described by Ostrelich, however, impose discrete thickness as well as weight requirements on the scale incorporating the same. While the film resistors themselves, like strain gauges, are less than 0.01" in thickness, there are a number of other mechanical and electrical components with discrete thickness and weight requirements that are required for the scale. In the embodiments described by Ostrelich, each transducer is mounted on a load-bearing base plate, and placed under pressure by means of a pair of pins separating the base plate from a loading plate and a spaced load-bearing cover plate, which bear the applied load and transmit it to the transducers. The three spaced load-bearing plates, and the intermediate force-transmitting pins, impose discrete thickness and weight requirements on the scale incorporating the same. The Ostrelich device additionally incorporates a fairly large battery cell for powering the electronic circuit and the LED, further increasing the thickness of the scale assembly.
Similar thickness and weight requirements are encountered with the small scales disclosed in Curchod U.S. Pat. No. 4,174,760; Schaenen U.S. Pat. No. 4,043,413; Hags U.S. Pat. No. 2,910,287; and Paddon et al. U.S. Pat. No. 4,363,368, for example.
These discrete thickness and weight requirements have been found in all known industrial, medical and personal scales produced to-date, thus making it difficult to transport them in bags while travelling, to lift them for a closer look, to store them in a cabinet or to hang them on the wall so as to keep them away from the floor. Even though many scales are advertised as being "portable", they are rarely small or light enough to be easily moved. For a scale to be truly portable, it should be considerably thinner and lighter than scales produced to date, e.g. 1/4" or less in thickness and 1 lb. or less in weight.